Communication systems using multiple-input multiple-output (MIMO) technology enable increased throughput for a given transmit power by exploiting spatial diversity at the transmitter and at the receiver. In a wireless MIMO system, gain and spatial diversity introduced by multiple antennas may increase spectral efficiency. Similarly, in a wire-line MIMO system, multiple physical channels may be combined with beam-forming to increase the total data throughput compared to a single-input single-output (SISO) system of identical transmit power.
Regulatory requirements typically impose limits on the maximum radiated emission of a communication system. These limits, which vary by regions, seek to limit the amount of power transmitted from a communication device to reduce interference on other devices operating in the same frequency band. For example, the HomePlug AV specification for North America limits the maximum transmit power spectral density to −50 dBm/Hz for bandwidth between 2-30 MHz for a device communicating over home power lines. For systems employing MIMO technology where there are multiple simultaneous transmit chains, the maximum transmit power applies to the overall emissions from the multiple transmit chains. Thus, the maximum transmit power may be distributed among the multiple transmit chains of the MIMO system, resulting in the transmit power on a single transmit chain to be less than the transmit power in a SISO system. If there is low data throughput on some of the transmit chains due to poor channel characteristics, the overall throughput of the multiple transmit chains may be less than the throughput of a SISO system. For example, if the transmit power is equally divided between two transmit chains of a MIMO HomePlug AV2 device, and if a first transmit chain experiences low throughput due to interference, the second transmit chain may not have sufficient diversity gain to compensate for the lower throughput of the first transmit chain. This may result in a lower overall data throughput in the MIMO device compared to a SISO device transmitting at maximum power through a single transmit chain Accordingly, there is a need to automatically compensate for different channel characteristics in the multiple transmit chains of a MIMO system to increase the overall data throughput while staying within the maximum transmit power imposed by regulatory requirements.